FIG. 1 illustrates an image capture system, such as a camera, having an imaging lens 10 with a focal length f and an imaging sensor 12. An object 14 at a distance do in front of the camera will be properly focused at the imaging sensor 12 at a distance di behind the lens if the relationship between do, di and f is                                           1                          d              o                                +                      1                          d              i                                      =                              1            f                    .                                    (        1        )            If an object 14 is moving at a constant velocity v (m/s) within the field of view (FOV) 16 of the scene, and the exposure time of the image capture system is Δt, then the object will appear smeared in the image over a distance s, given by                     s        =                                            d              i                                      d              o                                ⁢          v          ⁢                                           ⁢          Δ          ⁢                                           ⁢          t          ⁢                                           ⁢                                    (              meters              )                        .                                              (        2        )            If the imaging sensor 12 captures a digital image, then the object will be displaced by a distance s, measured in pixels, given by                               s          =                                                    d                i                                                              d                  o                                ⁢                p                                      ⁢            v            ⁢                                                   ⁢            Δ            ⁢                                                   ⁢            t            ⁢                                                   ⁢                          (              pixels              )                                      ,                            (        3        )            where p is the distance between adjacent detector elements in the digital detector array. Also, if the image capture system moves with respect to the scene at a velocity v during the exposure time of the image, then the entire scene within the FOV 16 will be smeared over a distance s.
The image smear can be reduced if the exposure time is decreased, but this will decrease the signal at the detector. If the signal is measured as the number of electrons produced by the object at the detector, then the signal is given by                                           s            object                    =                                                                      A                  detector                                ⁢                π                ⁢                                                                   ⁢                Δ                ⁢                                                                   ⁢                t                                            4                ⁢                                                      (                                          f                      ⁢                      #                                        )                                    2                                ⁢                hc                                      ⁢                                          ∫                                  λ                  mm                                                  λ                  max                                            ⁢                                                η                  ⁡                                      (                    λ                    )                                                  ⁢                                                      L                    object                                    ⁡                                      (                    λ                    )                                                  ⁢                                                      τ                    optics                                    ⁡                                      (                    λ                    )                                                  ⁢                λ                ⁢                                  ⅆ                  λ                                                                    ,                            (        4        )            where λ is the wavelength of light, τoptics is the transmittance of the optical system, Lobject is the spectral radiance of the object, η is the detector quantum efficiency, Adetector is the area of the detector element, f# is the optical system f-number, h is Planck's constant, and c is the speed of light. Random noise arises from elements that add uncertainty to the signal level of the target and is quantified by the standard deviation of its statistical distribution σnoise. The signal-to-noise ratio (SNR) of the image is given by                     SNR        =                                            mean              ⁢                                                           ⁢              object              ⁢                                                           ⁢              signal                                      signal              ⁢                                                           ⁢              deviation                                =                                                    s                object                                            σ                noise                                      .                                              (        5        )            Reducing the exposure time will decrease the image smear, but will also reduce the signal and the SNR of the image, generally resulting in an image that appears noisier.
Referring to FIG. 2, when a fast moving object 14 at a velocity v is imaged with a video capture system, the object 14 will be smeared by a distance s and the object 14 will be displaced between the two successive images, i.e. the first image frame 18 and the second image frame 20, by a distance Δd, given by                                           Δ            ⁢                                                   ⁢            d                    =                                                    d                i                                                              d                  o                                ⁢                p                                      ⁢                                          vt                clock                            ⁡                              (                pixels                )                                                    ,                            (        6        )            where tclock is the time interval between each successive image. Here a video capture system will be defined as an image capture system that acquires a sequence of images separated in time determined by the frame rate (FR). The frame rate is related to tclock by                               FR          =                                    1                              t                clock                                      ⁢                          (                              frames                ⁢                                  /                                ⁢                s                            )                                      ,                            (        7        )            and tclock≧Δt.
If the object displacement and the smear are large enough, then the utility of the video images will be reduced. Longer exposure times are desired to increase the SNR but may cause an unacceptable smear of the object. If the smear is large enough, identification of the object may not be possible. Motion smear above two pixels will cause a loss of information that is generally not recoverable. Lower frame rates are desired to reduce the amount of image data that must be captured, disseminated, and stored, but may not capture enough frames of a fast moving object before it passes through the camera's FOV. Traditional electronic video image capturing devices have a predefined exposure time, or shutter time, and readout each image at a predefined frame rate.
One technique used to acquire high-speed photographs that reduces the image motion smear without producing noisy images is to use a very fast illumination flash, such as a strobe, to decrease the effective exposure time. One such method proposed in U.S. Pat. No. 4,918,522 uses a short light impulse to image a quick-moving object by means of a video method. This is ideal in that it produces a high signal over a very short exposure time. If a strobe is used, then it must be synchronized with the frame rate of the video capture system. Unfortunately, it is not always convenient or even possible to have a high intensity flash source available while acquiring images. For example, image capture systems used for remote sensing can not use a flash source. It is desirable then to capture an image with the longest exposure time possible, to maximize the SNR, but that does not produce an undesirable amount of image motion smear.
With video capture systems, short exposure times can be used to minimize the image motion blur, and then successive frames can be added together to increase the signal. Fast moving objects in the scene can still produce artifacts in the reproduced image. The method proposed in U.S. Pat. No. 4,652,907 adaptively controls the parameters used in a recursive filter to sum the successive frames together to minimize the artifacts, but does not adaptively control the exposure time. Another method proposed in U.S. Pat. No. 4,882,629 adaptively controls the exposure time based on the signal level, but does not account for the motion of objects in the scene.
There is a need therefore for an image capture system that adaptively determines the best exposure time and frame rate based on the detected motion during the imaging sequence, thus keeping the image motion smear and the object displacement at or below a predetermined level.